Talk:Indianapolis Department Of Public Safety

Poor Voice Channel Decode on Indianapolis' Northwest Side

The problem scanner listeners experience is either scanner front-end overload, or, more likely, the RF properties of the system in that area (which have been specifically tuned to provide optimal performance to portable subscriber radios) are such that there is just too much destructive mixing in the front end of scanners to make the signal usable, despite the high RSSI. Think about it. You can go to the truck stops at I-465 and Harding on the south side, tune your radio to 27.185 MHz, and most of the time, your RSSI will be pegged. But is it a usable signal that you can actually understand?

There is obviously a similar, albeit more complex and fairly well documented issue with scanners monitoring linear simulcast modulation (LSM) CQPSK P25 digital voice in a simulcast cell of a true Project 25 Phase I (FDMA) trunked system. It has been shown through user experience that the best way to try to solve this problem is to attenuate the destructive multipath signals, usually by using a directional antenna pointed at the nearest transmit site in the simulcast cell. If the scanner then overloads, the attenuator may be called for, or experimenting with pointing the antenna in other directions to see where the best voice channel decoding can be found.

Just because you have high RSSI on the control channel doesn't mean that the scanner's decode rate of the control channel OSWs will be sufficient to decode all channel grant instructions, nor does it mean that you will automatically get good audio decode on the voice channels when the channel grant OSWs actually are decoded.

I have a Cushcraft/Signals commercial 800 MHz base station antenna at the house at about 15 feet up in the air. Its connected via Belden 9913 semi-flexible coax through a PolyPhaser on a ground bar to a PRO-2096 (with the newest available DSP firmware update) mounted to my radio desk. It does great for single-cell (non-simulcast) SAFE-T sites in Marion and surrounding counties, does particularly well for SAFE-T Boone County simulcast, and even receives Hamilton County EDACS at an acceptable level, which is pretty good considering my distance from the system, and, if I'm not mistaken it is an analog voice simulcast cell. However it is utterly useless for the MECA system. In fact, it can barely read control channel OSWs.

I have an identical antenna installed on a different mast about thirty yards away and about 15 feet higher that I use for control channel decode radios that feed Unitrunker, Trunk88, and Pro96Com. Specifically, it feeds an old PRO-93 and a PRO-96 that are combined on the same antenna. The PRO-93 is tapped, and feeds Unitrunker and Trunk88, while the PRO-96 dumps control channel via the PC/IF port through a GRE USB cable to Pro96Com. Even though the PRO-96 is essentially the same radio with the same DSP firmware updates as the PRO-2096, the PRO-96 gets much, much better OSW decoding on the MECA control channel that the 2096 does. What does this illustrate? When you're trying to monitor P25 simulcast with a scanner, its all about location, location, location.

I don't care if you have full-scale RSSI on the control channel and even have a halfway decent OSW decode rate. The fact is that voice channels in a LSM CQPSK P25 simulcast cell are subject to severe distortion when monitored on even the best scanners on the market today depending on where you are. Adjusting bandplan parameters in your scanner won't improve anything one bit, because the bandplan the system uses for 800 MHz P25 is standardized, and is automatically configured in the scanner without any additional input from the user.

The system was not built for scanners. It was built for subscriber radios. I can tell you that it is a damn good system and provides excellent coverage, even on a portable in places you wouldn't expect it. In my opinion, it is money well spent and provides a great, very reliable infrastructure for mission-critical public safety communications. The fact that scanners have trouble with it in some places is unfortunate, but its not a problem with the system by any stretch of the imagination.